Mathematical models of EMG force relationships in the presence of motion are being developed by both experimental and analytical methods. Extension of the knee is serving as the test vehicle. A coordinated series of studies are in progress to elucidate the major factors involved in the relationships between EMG and force. These relate to electrode placement and type (wire vs. surface), fiber length (joint position), fatigue, constant and variable velocity, periarticular tissue resistance, performance reliability, and antagonistic muscle action. The experimental results will be used in computer simulation with systematic identification methods to effect a synthesis of the mathematical model. As a prerequisite for optimum EMG acquisition, a series of studies have been completed to define the necessary amplifier and electrode specifications. These included impedance measurements, spectral analysis and computer simulation. Two amplifiers have been built (gain 1000, input impedance 10 to the 9th power ohms, common mode rejection rate 130 dB, minus 3dB frequencies 13 to 14000 Hz). Wire electrodes are of 5 microns diameter with 3 mm bared tips. Surface recording employs Beckman electrodes with 4 mm diameter and 50 mm inter-electrode spacing. Following a study of isometric and isokinetic maximum strength, a Nautalus knee unit is being modified with a custom lever series which approximates a cosine cube function and constant force springs for resistance.